Is "0.001%" the pot setting? That is not realistic.
Here is my first try on a sand state regulator for this thing. I know I still do have to get some resistor values right. But here is a first schem, just to get the idea:
Even two sides of a 6SN7 in parallel won't be happy driving 2.4K at higher levels. I'd use a 4:1 output, then try to make up the extra 6dB of loss somewhere else... perhaps use 1/2 6SL7 for the input stage, one side for each channel. And there's your three-bottle, two-channel preamp.
The level pot doesn't need to be 1M, and such a large value will cause high frequency rolloff, looking into the grid of the following stage
Don't put a lot of faith in those tube models in Circuitmaker. They tend to predict better performance (especially with large signals) than what you'll get in real life. Diddling around with a simulator is fun, but it's no substitute for real design work.
It's not going to be too tolerant of abuse as it stands, particularly in regards to the peak fault currents in the transistors (can you say PAFFFF goes the silicon?).
Be mindful of zener dissipation too.
Do you really need 600 ohm load performance?
If you must whack 600 ohms, you either need to flow DC in the output transformer or you need a bigger tube than these.